He continued to excel in track and field, capturing the 1989 Southeastern Conference indoor high jump championship with a jump of 7 ft 2½in. He also anchored the University of Tennessee 4x100 and 4x400 metres relays.

In American football, he earned All-Southeastern Conference Freshman First Team honors in 1987, and All-Southeastern Conference first team honors in 1990. He was also the MVP of the 1991 Hula Bowl, and finished his NCAA football career with 103 receptions for 1,547 yards and 16 touchdowns.

He was drafted by the Dallas Cowboys in the first round of the 1991 NFL Draft, in order to find a receiving partner for Michael Irvin. During his rookie year, he caught 20 passes for 326 yards and one touchdown, helping Dallas finish 11-5, its first winning season since 1985.

In 1993, Harper established himself as one of the best deep threats in the NFL with a 21.6 yards per catch average, and a total of 36 receptions for 777 yards and five touchdowns. He again had a key play in the NFC Championship against the San Francisco 49ers: with Troy Aikman knocked out of the game with a concussion, Dallas backup quarterback Bernie Kosar connected with Harper for a 42 yard touchdown. That year, Dallas returned to face the Bills in Super Bowl XXVIII during which Aikman completed a 16-yard pass to him to give the Cowboys a first-and-goal at the six early in the fourth quarter. That set up a one-yard-run by running back Emmitt Smith and helped seal the Bills fate as Dallas won 30-13.

Harper's best year came in 1994, when he led the league with a 24.9 yards-per-reception average, catching 33 passes for 821 yards and eight touchdowns. However, Dallas failed to take a third consecutive win, dropping out of Super Bowl contention with a 38-28 loss to the Super Bowl champions, the San Francisco 49ers.

In 1995, Harper signed a lucrative free agent contract with the Tampa Bay Buccaneers based on his big play ability and championship experience. However, although he had a career high 46 catches his first season in Tampa, he was not able to live up to his free agent billing.

He was released before the start of 1997 season. During his two years in Tampa he was slowed by injuries and played in only 25 games where he caught 65 passes for 922 yards and three touchdowns.